Method and device for optical information recording/reproduction

ABSTRACT

In an optical information reproducing method for reproducing information from an optical information recording medium of land/groove and low-to-high recording system having a substrate ( 1 ) and a recording layer ( 2 ), light having a wavelength of 390 to 440 nm is projected to the recording layer ( 2 ) through the substrate ( 1 ) with use of an objective lens having a numerical aperture of 0.65 to thereby reproduce the information, wherein assuming that a wavelength of the light is λ and an index of refraction of the substrate ( 1 ) in the wavelength λ is ns, a depth D of a portion (G′) of the recording layer ( 2 ) corresponding to the groove (G) with respect to a portion (L′) of the recording layer ( 2 ) corresponding to the land (L) satisfies a relation of λ/5.8 ns≦D≦λ/5 ns.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/510,460, filed Oct. 6, 2004 entitled ENDOSCOPE APPARATUSwhich is a U.S. National stage of application No. PCT/JP03/04438 filedon Apr. 8, 2003, which claims the benefit and priority of JapaneseApplication No. 2002-105106, filed in Japan on Apr. 8, 2002, thecontents of which are incorporated by this reference.

TECHNICAL FIELD

The present invention relates to an optical information recording mediumwith respect to which information is recorded/reproduced using lightsuch as laser light, further to an optical informationrecording/reproducing method and an optical informationrecording/reproducing device using the medium, particularly to anoptical information recording medium in which information is recordedboth in a portion corresponding to the inside of a guide groove fortracking and a portion corresponding to a portion between adjacent guidegrooves with respect to a recording layer disposed on the surface of asubstrate having a guide groove for tracking, and a method and devicefor optical information recording/reproducing using the medium.

BACKGROUND ART

As optical information recording mediums for use inrecording/reproducing information by irradiation with laser light, amagnetic optical disk (MO) or CD-R, or CD-RW, DVD-R, DVD-RAM, DVD-RW andthe like have been generally known. As means for raising recordingdensities in the optical information recording mediums, land/grooverecording has been known in which the recording is performed inrecording layer portions corresponding to both a flat portion (land)between adjacent guide grooves for tracking, the guide grooves beingformed into substantially circular shapes in parallel with one anotherin a substrate surface, and the inside of each guide groove(JP(A)-57-50330, JP(A)-9-73665, JP(A)-9-198716, JP(A)-10-64120 and thelike).

Moreover, in recent years, as a method of raising a recording density, atechnique has been proposed in which NA of an objective lens of anoptical head constituting a device for recording/reproducing informationis raised to about 0.85. When the NA is raised, a beam diameter incondensing laser light can be reduced, and therefore it is possible torecord/reproduce a finer mark. When the NA is raised in this manner,instead of applying the laser light from the side of a support substratehaving a thickness of 0.6 to 1.2 mm as in a conventional technique, alight-transmitting layer having a thickness of about 0.1 mm is formed onthe surface of the optical information recording medium in which theguide groove for tracking is formed, and the laser light can be appliedfrom the side of the light-transmitting layer to record/reproduceinformation.

Moreover, the raising of the recording density by shortening of awavelength of a laser light source has also been intensively studied. Ithas been expected that three times or higher recording density berealized by use of a bluish-purple semiconductor laser having awavelength of around 405 nm as compared with a red semiconductor laserfor use in recording/reproducing a conventional DVD.

On the other hand, to improve a signal quality of the opticalinformation recording medium, a low-to-high (L-H) recording system iseffective in which reflectance of the recording layer before therecording is lowered in such a manner as to raise the reflectance of therecording layer after the recording. In this system, since thereflectance of the recording layer before the recording is loweredbeforehand, a modulation degree can be raised, and therefore C/N can beraised as compared with a conventional high-to-low (H-L) recordingsystem for use in DVD-RAM or DVD-RW, whose reflectance of the recordinglayer after the recording lowers.

It is considered that these techniques are combined, that is, theland/groove recording is performed with respect to the opticalinformation recording medium of the L-H system having a high signalquality, using a short-wavelength light source such as a bluish-purplesemiconductor laser, and an optical head having a high NA, so that therecording density is significantly increased.

Additionally, one of large technique problems in performing theland/groove recording is leakage of a signal from an adjacent track,so-called crosstalk. When a pitch of the guide groove for tracking isreduced in order to increase the recording density, crosstalk componentsfrom information recorded in the adjacent track are increased, and it istherefore difficult to correctly reproduce the information in a targettrack (self track).

DISCLOSURE OF THE INVENTION

One of objects of the present invention is to provide an opticalinformation recording medium of an L-H system having a high signalquality, capable of performing high-density land/groove recording inwhich a crosstalk from an adjacent track is suppressed even in using ashort-wavelength light source or a high-NA optical head.

Moreover, another object of the present invention is to provide a methodand a device for recording/reproducing optical information using theoptical information recording medium.

In order to attain the above object, according to the present invention,there is provided an optical information reproducing method forreproducing information from an optical information recording medium ofland/groove and low-to-high recording system having a substrate and arecording layer, comprising the step of projecting light having awavelength of 390 to 440 nm to the recording layer through the substratewith use of an objective lens having a numerical aperture of 0.65 tothereby reproduce the information, wherein assuming that a wavelength ofthe light is λ and an index of refraction of the substrate in thewavelength λ is ns, a depth D of a portion of the recording layercorresponding to the groove with respect to a portion of the recordinglayer corresponding to the land satisfies a relation of λ/5.8 ns≦D≦λ/5ns.

In order to attain the above object, according to the present invention,there is provided an optical information reproducing device forreproducing information from an optical information recording medium ofland/groove and low-to-high recording system having a substrate and arecording layer, comprising a light source which emits light having awavelength of 390 to 440 nm and an objective lens having a numericalaperture of 0.65, wherein the light is projected to the recording layerthrough the substrate to thereby reproduce the information, and assumingthat a wavelength of the light is λ and an index of refraction of thesubstrate in the wavelength λ is ns, a depth D of a portion of therecording layer corresponding to the groove with respect to a portion ofthe recording layer corresponding to the land satisfies a relation ofλ/5.8 ns≦D≦λ/5 ns.

In order to attain the above object, according to the present invention,there is provided an optical information reproducing method forreproducing information from an optical information recording medium ofland/groove and low-to-high recording system having a substrate and arecording layer, comprising the step of projecting light having awavelength of 390 to 440 nm to the recording layer through the substratewith use of an objective lens having a numerical aperture of 0.65 tothereby reproduce the information, wherein assuming that: a quantity ofreflected light at a time when the light is applied to a non-recordingregion in which alternate arrangement of the land portion and the grooveportion is not formed is I₁; and quantities of the reflected light at atime when the light is applied to a portion of the recording layercorresponding to the groove and a portion of the recording layercorresponding to the land in an information non-recorded state on thesame conditions are I₂ and I₃, respectively, a value of R=0.5(I₂+I₃)/I₁falls within a fixed range.

In an aspect of the present invention, the value of R falls within arange of 0.5 to 0.75. In an aspect of the present invention, the valueof R falls within a range of 0.6 to 0.7.

In order to attain the above object, according to the present invention,there is provided an optical information reproducing device forreproducing information from an optical information recording medium ofland/groove and low-to-high recording system having a substrate and arecording layer, comprising a light source which emits light having awavelength of 390 to 440 nm and an objective lens having a numericalaperture of 0.65, wherein the light is projected to the recording layerthrough the substrate to thereby reproduce the information, and assumingthat: a quantity of reflected light at a time when the light is appliedto a non-recording region in which alternate arrangement of the landportion and the groove portion is not formed is I₁; and quantities ofthe reflected light at a time when the light is applied to a portion ofthe recording layer corresponding to the groove and a portion of therecording layer corresponding to the land in an information non-recordedstate on the same conditions are I₂ and I₃, respectively, a value ofR=0.5(I₂+I₃)/I₁ falls within a fixed range.

In an aspect of the present invention, the value of R falls within arange of 0.55 to 0.7. In an aspect of the present invention, the valueof R falls within a range of 0.6 to 0.7.

In order to attain the above object, according to the present invention,there is provided an optical information reproducing method forreproducing information from an optical information recording medium ofland/groove and low-to-high recording system having a substrate and arecording layer, comprising the step of projecting light having awavelength of 390 to 440 nm to the recording layer through the substratewith use of an objective lens having a numerical aperture of 0.65 tothereby reproduce the information, wherein assuming that a wavelength ofthe light is λ and an index of refraction of the substrate in thewavelength λ is ns, a depth D of a portion of the recording layercorresponding to the groove with respect to a portion of the recordinglayer corresponding to the land satisfies a relation of λ/5.8 ns≦D≦λ/5ns, and the sum of a width of the groove portion and a width of the landportion falls within a range of 0.5 to 1.2 μm.

In order to attain the above object, according to the present invention,there is provided an optical information reproducing device forreproducing information from an optical information recording medium ofland/groove and low-to-high recording system having a substrate and arecording layer, comprising a light source which emits light having awavelength of 390 to 440 nm and an objective lens having a numericalaperture of 0.65, wherein the light is projected to the recording layerthrough the substrate to thereby reproduce the information, and assumingthat a wavelength of the light is λ and an index of refraction of thesubstrate in the wavelength λ is ns, a depth D of a portion of therecording layer corresponding to the groove with respect to a portion ofthe recording layer corresponding to the land satisfies a relation ofλ/5.8 ns≦D≦λ/5 ns, and the sum of a width of the groove portion and awidth of the land portion falls within a range of 0.5 to 1.2 μm.

In order to attain the above object, according to the present invention,there is provided an optical information recording medium in which lightis projected in a spot to thereby record/reproduce information and inwhich at least a recording layer and a light-transmitting layer aredisposed in this order on a substrate having a guide groove for trackingof the spotted light and in which the light is projected in the spot tothe recording layer from the side of the light-transmitting layer torecord the information both in a first portion of the recording layercorresponding to the inside of the guide groove and a second portion ofthe recording layer corresponding to a flat portion between mutuallyadjacent guide grooves,

-   -   wherein assuming that a wavelength of the light is λ, and an        index of refraction of the light-transmitting layer in the        wavelength λ is nf, a depth d of the first portion with respect        to the second portion in the surface of the recording layer on        the side of the light-transmitting layer satisfies a relation of        λ/5.8 nf≦d≦λ/5 nf, and a reflectance of the recording layer        after the recording is larger than that before the recording.

In an aspect of the present invention, the depth d is substantiallyequal to a depth of the inside of the guide groove with respect to theflat portion between the guide grooves. In an aspect of the presentinvention, a dielectric layer exists between the substrate and therecording layer. In an aspect of the present invention, a reflectivefilm exists between the substrate and the dielectric layer. In an aspectof the present invention, a dielectric layer exists between therecording layer and the light-transmitting layer.

In order to attain the above object, according to the present invention,there is also provided an optical information recording medium in whichlight is projected in a spot to thereby record/reproduce information andin which at least a recording layer is disposed on a substrate having aguide groove for tracking of the spotted light and in which the light isprojected in the spot to the recording layer from the side of thesubstrate to record the information both in a first portion of therecording layer corresponding to the inside of the guide groove and asecond portion of the recording layer corresponding to a flat portionbetween mutually adjacent guide grooves,

-   -   wherein assuming that a wavelength of the light is λ, and an        index of refraction of the substrate in the wavelength λ is ns,        a depth D of the second portion with respect to the first        portion in the surface of the recording layer on the side of the        substrate satisfies a relation of λ/5.8 ns≦D≦λ/5 ns, and a        reflectance of the recording layer after the recording is larger        than that before the recording.

In an aspect of the present invention, the depth D is substantiallyequal to a depth of the inside of the guide groove with respect to theflat portion between the guide grooves. In an aspect of the presentinvention, a reflective film exists on the side of the recording layeropposite to the substrate. In an aspect of the present invention, adielectric layer exists between the substrate and the recording layer.

In order to attain the above object, according to the present invention,there is also provided an optical information recording medium in whichlight is projected in a spot to thereby record/reproduce information andin which at least a recording layer is disposed on a substrate having aguide groove for tracking of the spotted light and in which theinformation is recorded both in a first portion of the recording layercorresponding to the inside of the guide groove and a second portion ofthe recording layer corresponding to a flat portion between mutuallyadjacent guide grooves,

-   -   wherein assuming that: a quantity of reflected light at a time        when the light is applied to a non-recording region in which        alternate arrangement of the guide groove and the flat portion        between the guide grooves is not formed is I₁; and quantities of        the reflected light at a time when the light is applied to a        portion corresponding to the inside of the guide groove and a        portion corresponding to the flat portion between the guide        grooves in an information non-recorded state on the same        conditions are I₂ and I₃, respectively, a value of        R=0.5(I₂+I₃)/I₁ is 0.55 to 0.7, and a reflectance of the        recording layer after the recording is larger than that before        the recording.

In an aspect of the present invention, a dielectric layer exists betweenthe substrate and the recording layer, and a reflective film existsbetween the substrate and the dielectric layer. In an aspect of thepresent invention, a dielectric layer exists on the side of therecording layer opposite to the substrate, and a reflective film existson the side of the dielectric layer opposite to the recording layer.

In order to attain the above object, according to the present invention,there is also provided an optical information recording/reproducingmethod having the step of projecting light having a wavelength of 390 to440 nm in a spot to both a first portion and a second portion of arecording layer of the above optical information recording medium usingan objective lens having a numerical aperture of 0.8 to 0.9 to therebyrecord/reproduce information.

In order to attain the above object, according to the present invention,there is also provided an optical information recording/reproducingdevice having an optical head which projects light in a spot to both afirst portion and a second portion of a recording layer of the aboveoptical information recording medium, the optical head having asemiconductor laser which emits light having a wavelength of 390 to 440nm and an objective lens having a numerical aperture of 0.8 to 0.9.

It has heretofore been known well that when a groove depth is changed,the crosstalk in the land/groove recording changes. The present inventorhas found that in the L-H recording system, the groove depth capable ofreducing the crosstalk differs from that in the H-L recording system.FIG. 6 shows a relation between the groove depth and the crosstalk. InFIG. 6, the abscissa indicates a value of “a” in a case where the groovedepth is represented by λ/(a·n) assuming that the wavelength of laserlight is λ and an index of refraction of a light-transmitting layer or asubstrate existing on an incidence side of the laser light with respectto a recording layer is n, and the ordinate indicates an amplitude of acrosstalk signal. This means that the groove depth increases as “a”decreases. As shown in FIG. 6, the groove depth capable of reducing thecrosstalk is larger in the L-H system as compared with the H-L system.However, when the groove depth is increased, noises of the substrateremarkably increase. Therefore, even when the crosstalk can besuppressed, a signal quality in a target track (self track) drops, andhigh-density recording cannot be performed. When the groove depth isselected from the above-described proper range, that is, the value of“a” in FIG. 6 is set into a range of 5 to 5.8, the signal quality of theself track necessary for the high-density recording is secured, thecrosstalk from the adjacent track is suppressed, and a recordingcapacity can be increased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially enlarged sectional view of an optical informationrecording medium according to the present invention;

FIG. 2 is a schematic diagram showing a method and a device forrecording/reproducing information with respect to the opticalinformation recording medium according to the present invention;

FIG. 3 is a diagram showing one example of a jitter characteristic ofthe optical information recording medium according to the presentinvention;

FIG. 4 is a diagram showing one example of the jitter characteristic ofthe optical information recording medium whose reflectance drops afterrecording;

FIG. 5 is a diagram showing one example of the jitter characteristic ofthe optical information recording medium according to the presentinvention; and

FIG. 6 is a diagram showing a relation between a groove depth andcrosstalk of the optical information recording medium.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the present invention will be described hereinafterwith reference to the drawings.

FIG. 1 is a partially enlarged sectional view showing one embodiment ofan optical information recording medium according to the presentinvention. Guide grooves for tracking, extended in substantiallycircular shapes around a substrate center, are formed in the surface(upper surface) of a disc-shaped support substrate 1 having a thicknessof around 1.2 mm, and a flat portion (land) L is formed between mutuallyadjacent guide grooves. The inside (bottom portion) of the guide groovefor tracking is especially shown as a groove G. A depth (groove depth)of the groove G to the land L is D. A width of the land L is typicallysubstantially equal to that of the groove G preferably within an errorof 10%. Moreover, an arrangement pitch of the groove G is, for example,0.5 to 1.2 μm.

A dielectric layer 4 is formed on the upper surface of the substrate 1,a recording layer 2 in which optical information is recorded is formedon the dielectric layer 4, a dielectric layer 5 is formed on therecording layer 2, and a light-transmitting layer 3 having an index nfof refraction is formed on the dielectric layer 5. Laser light LB isapplied from a light-transmitting layer 3 side, and information isrecorded/reproduced with respect to the recording layer 2. Materialssuch as polycarbonate (PC) and aluminum (Al) can be used for thesubstrate 1. The recording layer 2 has a thickness of about 0.1 mm, andmay be a film of PC bonded by an ultraviolet cured resin or the like, ora layer formed of the ultraviolet cured resin having a thickness ofabout 0.1 mm. In case of the film of PC bonded, the thickness of the PCfilm is very large as compared with that of an ultraviolet cured resinbonding layer, and therefore the index of refraction of the PC film maybe used as the index nf of refraction of the light-transmitting layer 3.

For the recording layer 2, a material whose optical reflectance or phasechanges by irradiation with the laser light, for example, a known phasechange type recording material such as GeSbTe, a known photo refractivematerial or the like can be used. The recording layer 2 has aconcave/convex shape corresponding to a land/groove shape of the surfaceof the substrate 1, and a portion (i.e., first portion) G′ correspondingto the substrate groove G, and a portion (i.e., second portion) L′corresponding to the substrate land L are formed. A depth (groove depth)of the portion G′ corresponding to the groove with respect to theportion L′ corresponding to the land in the upper surface of therecording layer 2 is d. Typically, the thickness of the recording layer2 in the portion L′ corresponding to the land is equal to that in theportion G′ corresponding to the groove. Furthermore, since thethicknesses of the dielectric layers 4, 5 in the portion correspondingto the land are similarly equal to those in the portion corresponding tothe groove, the above-described groove depth d is substantially equal toD. Here, the groove depth d is selected in such a manner as to establishλ/5.8 nf≦d≦λ/5 nf assuming that a wavelength of the irradiation laserlight is λ. The thickness of the recording layer 2 is, for example, 10to 30 nm, preferably 10 to 20 nm. In addition to functions of protectivelayers, the dielectric layers 4, 5 also have functions capable ofrealizing a recording medium of an L-H system, when a layer constitution(including the thicknesses of the dielectric layers 4, 5) includingthese layers is appropriately set.

If necessary, a metal layer as a reflective film may be provided betweenthe upper surface of the substrate 1 and the dielectric layer 4.

The information is recorded/reproduced with respect to both the portionG′ corresponding to the groove and the portion L′ corresponding to theland of the recording layer 2 in the L-H recording system. To realizethe L-H recording system, the respective layers, the film thicknesses,and other layer constitutions are appropriately set in accordance with aknown design method.

FIG. 2 is a schematic diagram showing a method and a device forrecording/reproducing the information with respect to theabove-described optical information recording medium. An opticalinformation recording medium 10 rotates around a rotation center of avertical direction passing through the center of the medium. An opticalhead 20 constituting a recording/reproducing device is disposed abovethe recording medium 10. In the optical head 20, the laser light emittedfrom a semiconductor laser 21 which is a light source is applied in aspot shape to the portion G′ corresponding to the groove or the portionL′ corresponding to the land of the recording layer 2 of the recordingmedium 10 through a collimate lens 22 and an objective lens 23. Thereflected light from the recording medium 10 reaches an opticaldetection system 25 via the objective lens 23 and a beam splitter 24. Areproduction signal, a tracking signal or the like is obtained by theoptical detection system 25. A wavelength λ of the laser light appliedfrom the semiconductor laser 21 is, for example, 390 to 680 nm,preferably 390 to 440 nm. As the objective lens 23, a lens having alarge numerical aperture (NA), for example, of 0.6 to 0.9, preferably0.8 to 0.9 is used.

It is to be noted that the present invention is not limited to theapplication of the laser light from the side of the light-transmittinglayer 3, and the laser light may be applied from the side of thesubstrate 1. In this case, a light-transmitting substrate is used as thesubstrate 1. Since the thickness of the dielectric layer 4 of theportion corresponding to the land L is typically equal to that of theportion corresponding to the groove G as described above, the depth(groove depth) of the portion L′ corresponding to the land to theportion G′ corresponding to the groove is substantially D. The groovedepth D is selected in such a manner as to establish λ/5.8 ns≦D≦λ/5 nsassuming that a wavelength of the irradiation laser light is λ and anindex of refraction of the substrate 1 is ns. When the reflective layeris formed, the layer is disposed on the recording layer 2 via thedielectric layer 5. Also in this case, the information isrecorded/reproduced with respect to both the portion G′ corresponding tothe groove and the portion L′ corresponding to the land of the recordinglayer 2 in the L-H recording system.

Next, still another embodiment of the present invention will bedescribed. In the above-described embodiment, the relation between thegroove depth and the index of refraction of the light-transmitting layeror the substrate has been specified, and in the present embodiment, aquantity of reflected light of irradiation light is specified in aplurality of predetermined places of the recording medium. Accordingly,it is possible to record/reproduce with satisfactory jittercharacteristics.

Concretely, in the present embodiment, assuming that: the quantity ofthe reflected light is I₁ at a time when the laser light is applied in aspot to the recording layer in a non-recording region (e.g., regionpositioned inside or outside of a diametric direction with respect to aninformation recording region) having a layer constitution equivalent tothat of an information recording region in which the structure ofalternate arrangement of the lands/grooves of the recording medium 10 isformed as shown in FIG. 1, and having a uniform flat portion formed in asufficiently broad region as compared with a light beam spot diameterwithout any alternate arrangement of the lands/grooves; and quantitiesof the reflected light are I₂ and I₃ at a time when the laser light isapplied in the spots to the portion corresponding to the groove and theportion corresponding to the land, respectively, of the recording layerof the information recording region in an information non-recorded stateon the same conditions, a value of R=0.5(I_(2+I) ₃)/I₁ is 0.55 to 0.7.It has been found that this is effective in realizing satisfactoryjitter characteristics. R is preferably 0.6 to 0.7.

Here, as the quantity of the reflected light in the informationrecording region, an average value of the quantities of the reflectedlight from the portion corresponding to the groove and the portioncorresponding to the land is adopted. A main reason for this is that acase where the width of the groove is different from that of the landhas been considered. In the land/groove recording, the width of thegroove is equal to that of the land, and the value of I₂ issubstantially equal to that of I₃. This is typical, but there is also acase where the width of the groove is different from that of the landand the value of I₂ is different from that of I₃ because of amanufacturing error and the like, and this case is handled.

According to the present embodiment, instead of the relation between thegroove depth and the index of refraction of the light-transmitting layeror the substrate, a relation between the quantities of the reflectedlight in a plurality of places where the measuring is simple is defined,and accordingly a similar function/effect can be exerted.

The present invention will be further described hereinafter inaccordance with examples.

EXAMPLE 1

A disc-shaped PC substrate having a thickness of 1.1 mm was used as asubstrate, a 100 nm thick Al reflective film, a 40 nm thick ZnS—SiO₂dielectric layer, a 15 nm thick GeSbTe recording layer, and a 100 nmthick ZnS—SiO₂ dielectric layer were stacked in order by sputtering,respectively, and a recording medium (disk) was obtained. It is to benoted that a substrate whose pitch of a guide groove was 0.56 μm andwhose depth of the guide groove was 35 nm to 55 nm was used as the PCsubstrate. Furthermore, a 0.1 mm thick PC film was bonded by anultraviolet cured resin. An index (nf) of refraction of the PC film in awavelength of 400 nm was 1.6.

After initializing (crystallizing) the disk, the disk was rotated at alinear speed of 5.1 m/s, laser light was applied from the side oppositeto that of the substrate using an optical head comprising a laser lightsource having a wavelength (λ) of 400 nm and an objective lens havingNA=0.85, and land/groove recording was performed on a linear densitycondition of 0.116 μm/bit in an L-H recording system to measure jitters.In the disk, a reflectance before the recording (informationnon-recorded state) was 6%, and that after the recording (informationrecorded state) was 20%.

Measurement results of the jitters are shown in FIG. 3. FIG. 3 showsboth a jitter in a case where data is not recorded in an adjacent track(shown as without X.T.) and a jitter in a case where the data isrecorded in the adjacent track (shown as with X.T.). The jitter in theself track in a case where the adjacent track is not recorded and thereis not any crosstalk is satisfactory, when a groove depth is smaller.When the depth is larger than 50 nm, noises attributed to the substrateincrease, and therefore the jitter in the self track is remarkablydeteriorated. On the other hand, in a case where the adjacent track isalso recorded, when the groove depth is smaller than 43 nm, the jitteris remarkably deteriorated by an influence of the crosstalk. When thegroove depth is set in a range of 43 to 50 nm, that is, in a range ofλ/5.8 nf or more and λ/5 nf or less, it is possible to obtainsatisfactory jitter characteristics including the crosstalk.

COMPARATIVE EXAMPLE 1

A disc-shaped PC substrate having a thickness of 1.1 mm was used as asubstrate, a 100 nm thick Al reflective film, a 15 nm thick ZnS—SiO₂dielectric layer, a 15 nm thick GeSbTe recording layer, and a 45 nmthick ZnS—SiO₂ dielectric layer were stacked in order by sputtering,respectively, and a recording medium (disk) was obtained. It is to benoted that a substrate whose pitch of a guide groove was 0.56 μm andwhose depth of the guide groove was 35 nm to 55 nm was used as the PCsubstrate. Furthermore, a 0.1 mm thick PC film was bonded by anultraviolet cured resin. An index of refraction of the PC film in awavelength of 400 nm was 1.6.

After initializing (crystallizing) the disk, the disk was rotated at alinear speed of 5.1 m/s, laser light was applied from the side oppositeto that of the substrate using an optical head comprising a laser lightsource having a wavelength of 400 nm and an objective lens havingNA=0.85, and land/groove recording was performed on a linear densitycondition of 0.116 μm/bit in an H-L recording system to measure jitters.In the disk, a reflectance before the recording was 18%, and that afterthe recording was 2%.

Measurement results of the jitters are shown in FIG. 4. The jitter inthe self track in a case where an adjacent track is not recorded issatisfactory, when a groove depth is smaller. When the depth is largerthan 50 nm, the jitter in the self track is remarkably deteriorated. Onthe other hand, in a case where the adjacent track is also recorded,when the groove depth is smaller than 37 nm, the jitter is remarkablydeteriorated by an influence of crosstalk. When the groove depth islarger than 43 nm, an increase of the jitter by the influence of thecrosstalk cannot be ignored.

As seen from comparison of Comparative Example 1 with Example 1(comparison of FIG. 3 with FIG. 4), the H-L recording system isdifferent from the L-H recording system in a preferable groove depth,and additionally the jitter can be further reduced in the L-H recordingsystem.

EXAMPLE 2

A disc-shaped PC substrate having a thickness of 1.1 mm was used as asubstrate, a 100 nm thick Al reflective film, a 45 nm thick ZnS—SiO₂dielectric layer, a 13 nm thick GeSbTe recording layer, and a 110 nmthick ZnS—SiO₂ dielectric layer were stacked in order by sputtering,respectively, and a recording medium (disk) was obtained. It is to benoted that a substrate whose pitch of a guide groove was 0.6 μm andwhose depth of the guide groove was 38 nm to 60 nm was used as the PCsubstrate. Furthermore, a 0.1 mm thick PC film was bonded by anultraviolet cured resin. An index of refraction of the PC film in awavelength of 432 nm was 1.6.

After initializing (crystallizing) the disk, the disk was rotated at alinear speed of 5.1 m/s, laser light was applied from the side oppositeto that of the substrate using an optical head comprising a laser lightsource having a wavelength (λ) of 432 nm and an objective lens havingNA=0.85, and land/groove recording was performed on a linear densitycondition of 0.125 μm/bit in an L-H recording system to measure jitters.In the disk, a reflectance before the recording was 5%, and that afterthe recording was 21%.

Measurement results of the jitters are shown in FIG. 5. The jitter inthe self track in which the adjacent track is not recorded issatisfactory, when a groove depth is smaller. When the depth is largerthan 54 nm, the jitter in the self track is remarkably deteriorated. Onthe other hand, in a case where the adjacent track is also recorded,when the groove depth is smaller than 46 nm, the jitter is remarkablydeteriorated by an influence of the crosstalk. When the groove depth isset in a range of 46 to 54 nm, that is, in a range of λ/5.86 nf or moreand λ/5 nf or less, it is possible to obtain satisfactory jittercharacteristics including the crosstalk.

EXAMPLE 3

A PC substrate having a thickness of 1.1 mm was used as a substrate, a100 nm thick Al reflective film, a 65 nm thick ZnS—SiO₂ dielectriclayer, a 13 nm thick GeSbTe recording layer, and a 150 nm thick ZnS—SiO₂dielectric layer were stacked in order by sputtering, respectively, anda recording medium (disk) was obtained. It is to be noted that asubstrate whose pitch of a guide groove was 1.0 μm and whose depth ofthe guide groove was 62 nm to 98 nm was used as the PC substrate.Furthermore, a 0.1 mm thick PC film was bonded by an ultraviolet curedresin. An index of refraction of the PC film in a wavelength of 660 nmwas 1.58.

After initializing (crystallizing) the disk, the disk was rotated at alinear speed of 5.1 m/s, laser light was applied from the side oppositeto that of the substrate using an optical head comprising a laser lightsource having a wavelength (λ) of 660 nm and an objective lens havingNA=0.85, and land/groove recording was performed on a linear densitycondition of 0.21 μm/bit in an L-H recording system to measure jitters.In the disk, a reflectance before the recording was 6%, and that afterthe recording was 25%.

As a result of checking of a groove depth by which jittercharacteristics including crosstalk are satisfactory in the same manneras in Examples 1 and 2, it has been found that when the groove depth isset in a range of 72 to 83 nm, that is, in a range of λ/5.8 nf or moreand λ/5 nf or less, it is possible to obtain satisfactory jittercharacteristics including the crosstalk.

EXAMPLE 4

A PC substrate having a thickness of 0.6 mm was used as a substrate, a100 nm thick ZnS—SiO₂ dielectric layer, a 15 nm thick GeSbTe recordinglayer, a 40 nm thick ZnS—SiO₂ dielectric layer, and a 100 nm thickreflective film were stacked in order by sputtering, respectively, and arecording medium (disk) was obtained. After forming the Al reflectivefilm, the film was bonded to a 0.6 mm thick dummy PC substrate on whichany film was not formed by an ultraviolet cured resin, and thereafterevaluation was performed. It is to be noted that a substrate whose pitchof a guide groove was 0.7 μm and whose depth of the guide groove was 35nm to 55 nm was used as the PC substrate. An index of refraction (ns) ofthe PC substrate in a wavelength of 400 nm was 1.6.

After initializing (crystallizing) the disk, the disk was rotated at alinear speed of 5.1 m/s, laser light was applied from the side of thesubstrate using an optical head comprising a laser light source having awavelength (λ) of 400 nm and an objective lens having NA=0.65, andland/groove recording was performed on a linear density condition of0.152 μm/bit in an L-H recording system to measure jitters. In the disk,a reflectance before the recording was 6%, and that after the recordingwas 20%.

As a result of the measuring of the jitter, in the same manner as inExample 1, it has been found that when the groove depth is set in arange of 43 to 50 nm, that is, in a range of λ/5.8 ns or more and λ/5 nsor less, it is possible to obtain satisfactory jitter characteristicsincluding the crosstalk.

The results of Examples 1 to 4 described above are put together in thefollowing table 1. TABLE 1 Example 1 Example 2 Example 3 Example 4Wavelength λ (nm) 400 432 660 400 of laser light Index nf or ns of 1.61.6 1.58 1.6 refraction of PC film or PC substrate in wavelength λAllowable groove 43 46 72 43 depth lower limit value d1 (nm) d1/(λ/nf)or 1/5.8 1/5.86 1/5.8 1/5.8 d1/(λ/ns) Allowable groove 50 54 83 50 depthupper limit value d2 (nm) d2/(λ/nf) or 1/5   1/5   1/5   1/5   d2/(λ/ns)

As seen from Table 1, when the groove depth is set in a range of λ/5.8nf (or λ/5.8 ns) or more and λ/5 nf (or λ/5 ns) or less in anywavelength, satisfactory jitter characteristics including the crosstalkare obtained.

EXAMPLE 5

When I₁, I₂ and I₃ described above were measured using the optical headused in Example 4 with respect to the optical disk used in Example 4, avalue of R=0.5 (I₂+I₃)/I₁ was calculated, and a relation between thisvalue and a jitter was measured, results shown in Table 2 were obtained.It is to be noted that the jitter was obtained by performing recordingwith respect to all seven adjacent portions including portionscorresponding to lands and portions corresponding to grooves of arecording layer of each disk, and obtaining an average of measurementsin the portion corresponding to a middle land and the portioncorresponding to a middle groove. TABLE 2 R = 0.5(I₂ + I₃)/I₁ 0.5 0.550.6 0.7 0.75 0.7 Jitter (%) 12 9.6 9 9.3 11 14

As shown in Table 2, satisfactory characteristics were obtained in arange of R of 0.55 to 0.7, and most satisfactory characteristics wereobtained especially in a range of 0.6 to 0.7.

Furthermore, also with respect to the optical disks used in Examples 1to 3, I₁, I₂ and I₃ were measured using the optical head used in eachexample, a value of R=0.5(I_(2+I) ₃)/I₁ was calculated, a relationbetween this value and the jitter was checked, and then it was confirmedthat the value of R was in a range of 0.55 to 0.7 in the optical diskhaving satisfactory jitter characteristics.

That is, it has been found that the satisfactory characteristics areobtained in a range of R of 0.55 to 0.7 in the optical informationrecording medium of the L-H recording system, when the NA of theobjective lens of the optical head, the laser light wavelength, thethickness of the recording layer, and the index of refraction of thelight-transmitting layer or the substrate have various values.

It is to be noted that in the present example, the land width was equalto the groove width, but when the similar measuring was performed in acase where the land width was different from the groove width by about10%, similar results were obtained with respect to a preferable range ofR.

COMPARATIVE EXAMPLE 2

An optical information recording medium of an H-L recording system wasprepared using the same PC substrate as that of Example 4, and arelation between R and jitter was checked using the same optical head asthat of Example 4. As a result, it was confirmed that a range of R inwhich the jitter was satisfactory was 0.7 to 0.8, and the preferablerange of R was different from that in the L-H recording system. A valueof the jitter in the preferable range of R was larger as compared withExample 5.

Therefore, it has been confirmed that the L-H recording system issuitable for raising a recording density.

EXAMPLE 6

Suppression of crosstalk depends not only on the groove depth but alsoon a beam diameter or beam size of laser light for performing theinformation recording/reproducing. The beam diameter of the laser lightused in Examples 4 and 5 is 0.55 μm, which is typical in the opticalhead of wavelength of 400 nm and numerical aperture of the objectivelens of 0.65. The beam diameter is set in consideration of a parameter,i.e. rim intensity. The beam diameter is typically set so as to obtainthe rim intensity of about 60%. The rim intensity is defined as a ratioin intensity of laser light at the periphery of the objective lensrelative to the laser light at the center of the objective lens. As therim intensity becomes higher, the loss of light, i.e. eclipse, in theobjective lens becomes greater and accordingly the beam diameter can bereduced. However, in this case, power of the laser light focused on theoptical recording medium lowers because of greater loss of the light,and therefore power of the laser light emitted from the light sourcemust be strengthened so as to obtain necessary power of the laser lighton the optical recording medium. On the other hand, as the rim intensitybecomes lower, the power of the laser light emitted from the lightsource can be used with high efficiency although the beam diameterenlarges. In the actual optical head, the rim intensity may be set so asto realize the power of the laser light necessary for recording and thebeam diameter for obtaining good recording/reproducing performances. Therim intensity is typically set to about 60%.

In this example, there was examined how the most preferable groove depthwas changed by changing beam diameter. The beam diameter was changed bychanging the rim intensity. With use of the same optical disk as that ofExample 4, there were examined changes in R and jitter when the groovedepth was changed. The preferable range of groove depth set in thepresent invention, i.e. λ/5.8 ns to λ/5 ns, was 43 to 50 nm.

The results are shown in the following tables 3, 4 and 5. The preferablerange of groove depth within which the jitter is lower than 10% is 43 to52 nm for the case of beam diameter of 0.55 μm. However, for the casesof beam diameter of 0.51 μm and 0.6 μm, although relatively lower jittercan be obtained within the range of groove depth is 43 to 52 nm, thepreferable range of groove depth is 50 to 52 nm and 43 to 50 nm,respectively, which differ from the preferable range of groove depth forthe case of beam diameter of 0.55 μm. That is, the preferable range ofgroove depth varies in accordance with the beam diameter. The mostpreferable groove depth is 50-52 nm, 50 nm and 43 nm for the cases ofbeam diameter of 0.51 μm, 0.55 μm and 0.6 μm, respectively. However,from relations between R and jitter, it can be understood that thejitter is lower independent of the beam diameter when the range of R iswithin 0.5 to 0.75. In particular, it can be understood that thepreferable range of R within which the jitter is lower than 10% is 0.55to 0.7 independent of the beam diameter. Accordingly, the parameter R ismore descriptive of the recording/reproducing performances. TABLE 3 Beamdiameter: 0.51 μm Groove depth (nm) 35 40 43 50 52 55 R = 0.5(I₂ +I₃)/I₁ 0.84 0.79 0.74 0.69 0.58 0.53 Jitter (%) 15 13.5 10.5 9.2 9.210.3

TABLE 4 Beam diameter: 0.55 μm Groove depth (nm) 35 40 43 50 52 55 R =0.5(I₂ + I₃)/I₁ 0.8 0.75 0.7 0.6 0.55 0.5 Jitter (%) 14 11 9.3 9 9.6 12

TABLE 5 Beam diameter: 0.6 μm Groove depth (nm) 35 40 43 50 52 55 R =0.5(I₂ + I₃)/I₁ 0.76 0.71 0.67 0.57 0.52 0.48 Jitter (%) 11.3 10.2 9.19.3 11 14

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, it is possibleto perform land/groove recording with respect to an L-H system opticalinformation recording medium having a high signal quality using ahigh-NA optical head with a high recording density.

1. An optical information reproducing method for reproducing informationfrom an optical information recording medium of land/groove andlow-to-high recording system having a substrate and a recording layer,comprising the step of projecting light having a wavelength of 390 to440 nm to the recording layer through the substrate with use of anobjective lens having a numerical aperture of 0.65 to thereby reproducethe information, wherein assuming that a wavelength of the light is λand an index of refraction of the substrate in the wavelength λ is ns, adepth D of a portion of the recording layer corresponding to the groovewith respect to a portion of the recording layer corresponding to theland satisfies a relation of λ/5.8 ns≦D≦λ/5 ns.
 2. An opticalinformation reproducing device for reproducing information from anoptical information recording medium of land/groove and low-to-highrecording system having a substrate and a recording layer, comprising alight source which emits light having a wavelength of 390 to 440 nm andan objective lens having a numerical aperture of 0.65, wherein the lightis projected to the recording layer through the substrate to therebyreproduce the information, and assuming that a wavelength of the lightis λ and an index of refraction of the substrate in the wavelength λ isns, a depth D of a portion of the recording layer corresponding to thegroove with respect to a portion of the recording layer corresponding tothe land satisfies a relation of λ/5.8 ns≦D≦λ/5 ns.
 3. An opticalinformation reproducing method for reproducing information from anoptical information recording medium of land/groove and low-to-highrecording system having a substrate and a recording layer, comprisingthe step of projecting light having a wavelength of 390 to 440 nm to therecording layer through the substrate with use of an objective lenshaving a numerical aperture of 0.65 to thereby reproduce theinformation, wherein assuming that: a quantity of reflected light at atime when the light is applied to a non-recording region in whichalternate arrangement of the land portion and the groove portion is notformed is I₁; and quantities of the reflected light at a time when thelight is applied to a portion of the recording layer corresponding tothe groove and a portion of the recording layer corresponding to theland in an information non-recorded state on the same conditions are I₂and I₃, respectively, a value of R=0.5(I_(2+I) ₃)/I₁ falls within afixed range.
 4. The optical information reproducing method according toclaim 3, wherein the value of R falls within a range of 0.5 to 0.75. 5.The optical information reproducing method according to claim 4, whereinthe value of R falls within a range of 0.6 to 0.7.
 6. An opticalinformation reproducing device for reproducing information from anoptical information recording medium of land/groove and low-to-highrecording system having a substrate and a recording layer, comprising alight source which emits light having a wavelength of 390 to 440 nm andan objective lens having a numerical aperture of 0.65, wherein the lightis projected to the recording layer through the substrate to therebyreproduce the information, and assuming that: a quantity of reflectedlight at a time when the light is applied to a non-recording region inwhich alternate arrangement of the land portion and the groove portionis not formed is I₁; and quantities of the reflected light at a timewhen the light is applied to a portion of the recording layercorresponding to the groove and a portion of the recording layercorresponding to the land in an information non-recorded state on thesame conditions are I₂ and I₃, respectively, a value of R=0.5(I_(2+I)₃)/I₁ falls within a fixed range.
 7. The optical information reproducingdevice according to claim 6, wherein the value of R falls within a rangeof 0.55 to 0.7.
 8. The optical information reproducing device accordingto claim 7, wherein the value of R falls within a range of 0.6 to 0.7.9. An optical information reproducing method for reproducing informationfrom an optical information recording medium of land/groove andlow-to-high recording system having a substrate and a recording layer,comprising the step of projecting light having a wavelength of 390 to440 nm to the recording layer through the substrate with use of anobjective lens having a numerical aperture of 0.65 to thereby reproducethe information, wherein assuming that a wavelength of the light is λand an index of refraction of the substrate in the wavelength λ is ns, adepth D of a portion of the recording layer corresponding to the groovewith respect to a portion of the recording layer corresponding to theland satisfies a relation of λ/5.8 ns≦D≦λ/5 ns, and the sum of a widthof the groove portion and a width of the land portion falls within arange of 0.5 to 1.2 μm.
 10. An optical information reproducing devicefor reproducing information from an optical information recording mediumof land/groove and low-to-high recording system having a substrate and arecording layer, comprising a light source which emits light having awavelength of 390 to 440 nm and an objective lens having a numericalaperture of 0.65, wherein the light is projected to the recording layerthrough the substrate to thereby reproduce the information, and assumingthat a wavelength of the light is λ and an index of refraction of thesubstrate in the wavelength λ is ns, a depth D of a portion of therecording layer corresponding to the groove with respect to a portion ofthe recording layer corresponding to the land satisfies a relation ofλ/5.8 ns≦D≦λ/5 ns, and the sum of a width of the groove portion and awidth of the land portion falls within a range of 0.5 to 1.2 μm.